This invention relates to oil and gas well drilling. More particularly, this invention relates to directional drilling of boreholes using a bent sub or bent motor housing and an offset stabilizer in combination with a downhole motor such as a turbodrill or a positive displacement Moineau (PDM) motor.
Directional drilling, that is, the change in direction of drilling in the bore of subterranean well is well known and has been practiced for many years. In many instances, the change in direction is to produce a straightening of the well due to the deflection of the rotary drill from the desired direction by a particular rock strata. In other instances, the change in direction is intentional in order to reach a formation that is laterally displaced from the existing location of the bore hole.
One of the most common expedients for changing the direction of drilling has been the insertion in the drilling or work string, at a point above a downhole motor which drives the rotary drill bit, of an apparatus which is called a bent sub. Such bent subs are rigidly connected at one end to the work string and have their other connecting end angularly disposed relative to the axis of the work string to which they are connected, so that when the motor and supported drill bit are rigidly connected thereto, the axis of the drill bit will be angularly inclined relative to the axis of the well bore existing prior to insertion of the bent sub.
Alternatively, the required bend for changing the drilling direction may be incorporated directly in the downhole motor housing. Such apparatuses are known as bent motor housings and work in a substantially similar fashion to bent subs.
Still another method of changing the direction of drilling is the incorporation of an eccentric or offset stabilizer at the lower end of the drill string below the downhole motor and near the drill bit (defining a near bit stabilizer).
In addition, it has been found that directional drilling capabilities can be further enhanced by combining an offset near bit stabilizer with a bent sub or bent motor housing. However, this enhancement is maximized only if the offset stabilizer is radially aligned with the angle in the bent sub or bent motor housing. Unfortunately, because of the threaded connections between the drill string segments and the offset stabilizer, only random angular orientations with respect to the bent sub or bent motor housings and the offset stabilizer are produced. This is a significant drawback to the overall efficiency of directional drilling when an offset stabilizer is used in combination with a bent sub or bent motor housing.